Not applicable.
This invention relates generally to radar signal processing and more particular to an apparatus and method for determining the range and/or radial velocity of a target.
As is known in the art, radar systems, such as pulse Doppler radar systems, are used to determine the range and/or relative velocity (i.e., Doppler velocity) of an object. Radar pulses are transmitted at a rate referred to as the pulse repetition frequency (PRF). The time interval between successive pulses is referred to as the pulse repetition interval (PRI). During a predetermined time after pulse transmission, radar return signals are sampled, or gated, by the radar system. That is, based on the difference in time between pulse transmission and the time which the sample is taken, each one of the samples corresponds to a range, or distance, between the radar system and the object producing the sampled return. The process is referred to as range gating, where each time a sample is taken represents a range cell, or gate, of the return produced by the object at the range corresponding to the time at which the sample is taken.
In applications where there is a relative velocity (i.e., Doppler velocity) between the radar system and the object, in order to track the object, the time at which the radar return sample is sampled after pulse transmission varies in relation to the relative velocity between the radar system and the object. Thus, if the object is moving away from the radar system, the time at which the radar return is sampled relative to the time the radar pulse was transmitted, must increase from radar pulse to radar pulse at a rate proportional to the relative velocity, or Doppler velocity, between the radar system and the object. In like manner, if the object is moving towards from the radar system, the time at which the radar return is sampled relative to the time the radar pulse was transmitted must decrease from radar pulse to radar pulse at a rate proportional to the Doppler velocity between the radar system and the object.
In order to determine the Doppler velocity of the object, the radar returns from a plurality of transmitted radar pulses are processed. More particularly, each set of radar returns from a plurality of consecutively transmitted radar pulses is referred to as a dwell. The radar system produces a plurality of consecutive dwells. For each dwell, the radar system determines a plurality of Doppler frequency windows at each of a plurality of contiguous range gates.
In other words, for each dwell, the radar system determines a plurality of Doppler frequency windows at each of a plurality of contiguous range gates to provide a Doppler matrix. If there are N pulses, there are N frequency windows (or bins). The number of range gates corresponds to the number of input samples taken from each PRI. If M input samples are extracted from each of the N PRIs, M discrete Fourier transforms (DFTs) are performed to produce a matrix of M ranges and N frequency windows.
Fine Doppler velocity resolution generally requires a large number of radar returns per dwell (i.e., a relatively large data collection period). For objects having relatively high Doppler velocities, this data collection period translates into a time period during which the object to radar system range can experience a large change. If the range accuracy is less than the object movement over the dwell, some type of dynamic range gate adjustment (i.e., range gate positioning system) is required in order to maintain the range to the object in the middle of each dwell and thereby enable the radar system to track the range to the object with maximum signal to noise ratio.
Various systems have used range xe2x80x9cwalkxe2x80x9d compensation or velocity aiding techniques to move range gates during a dwell to prevent the object from passing through the xe2x80x9cwindowxe2x80x9d (i.e., time duration) of the range gate. For example, U.S. Pat. No. 5,943,003 entitled xe2x80x9cRadar Systemxe2x80x9d, issued on Aug. 24, 1999, being assigned to the same assignee as the present invention and incorporated herein by reference teaches one improved technique to determine the range and/or relative velocity of a target. In pulsed radar applications, it is often desirable to use dwell integration and radio frequency signals having wide frequency diversity, i.e. using radar signals having a carrier frequency which varies over a wide frequency range to determine the range and radial velocity of a target. It is also desirable to use pulse compression techniques to direct additional energy at the target using longer signal pulses while retaining the resolution of shorter pulses. Radar signals are transmitted as a series of pulses, and each of these pulses along with a time interval allocated to receive the return signals form a coherent processing interval (CPI). Each CPI can use different carrier frequencies with a dwell period or dwell cycle being described as a sequence of CPIs. Multiple DFTs of received signals during each CPI are commonly employed as a means to enhance target signal to interference ratios. The transforms are, however, difficult to mathematically integrate. The radio frequency (RF) shifts among the CPIs cause the target""s Doppler frequency to vary. During processing the received signals are sorted by frequency into a plurality of filter bins and assigned a filter bin number. When frequency diversity (i.e. varying the carrier frequency of the series of pulses) is used to detect targets with conventional processing techniques, the variation in Doppler frequency due to different carrier frequencies is referred to as bin shift.
For a pulsed-Doppler radar, it is difficult to non-coherently integrate rangexe2x80x94Doppler matrices of different carrier frequencies because the Doppler bins align in frequency and not in terms of the parameter to be measured, for example the incoming velocity. Therefore, algorithms are required to determine how values in the various Doppler bins should be non-coherently added. This alignment problem is generally referred to as a xe2x80x9cbin shiftxe2x80x9d problem. Additionally, the Doppler response is cyclic, depending on the dwell time and the radar pulse repetition frequency (PRF). This can lead to a Doppler ambiguity for a target since the aliases are measured along with the true Doppler when using a high PRF. Adding a Doppler phase correction is sometimes desired in processing the radar signals, and not knowing the correct ambiguous Doppler signal leads to an incorrect phase correction which can cause range-rate and range errors. Conventional systems require post-processing techniques to correct bin shift problems in order to provide non-coherent integration (NCI), Other problems such as target spread remain after post processing and required complicated phase correction techniques for pulse compression exist in conventional systems.
It would therefore be desirable to enhance signal detectability by non-coherent integration without the need for extensive post processing of the radar return signals to align signal Doppler frequencies.
In accordance with the present invention, a method for processing pulse Doppler radar signals to detect a target includes transmitting radar signals from a radar system having a predetermined varying frequency, receiving signals within a frequency band, including a target return signal having a frequency indicative of the velocity of the target, and transforming the target return signal using a Fourier transform having a variable frequency scale. With such a technique, signal detectability is enhanced by non-coherent integration without the need for extensive post processing of the radar return signals to align signal Doppler frequencies. The inventive technique takes advantage of integrating multiple dwells to achieve higher sensitivity and uses frequency diversity to enhance target cross section and mitigate RF multipath interference. This technique solves the bin shift problem by effectively changing the DFT equation. The modified equation results in a xe2x80x9cTarget Velocityxe2x80x9d normalized version of the signal""s spectral content, thus lining up the target return in the same DFT bin for each RF carrier frequency used.
In accordance with a further aspect of the present invention, the technique further includes aligning a radial velocity matrix for non-coherent integration, identifying a plurality of detections which occur from Doppler aliases, and removing Doppler aliases from the plurality of detections. With such a technique, velocity ambiguities in systems using relatively high PRFs are resolved. The modified technique Fourier transforms the plurality of detections into radial velocity space, which can be set for any desired minimum and maximum limits. Radial velocities appear in the range/Doppler matrix cyclically with different periods for different carrier frequencies. Only at the correct range-rate will the received signals align. Any detections which occur from Doppler aliases are identified and removed from a detection list. Since the range of the desired range-rates, both minimum and maximum, are not limited in this process, the unambigous range-rate zone is limited only by processing time. Therefore, any Doppler phase correction can be made for the correct Doppler, reducing the resultant range and radial velocity errors.
In one embodiment, the modified DFT technique transforms time samples into radial velocity space instead of Doppler frequency space. The resulting range by radial velocity matrices are pre-aligned for non-coherent integration. The limits of the radial velocity are not restricted by the time of the dwell or the number of pulses in the coherent processing interval (CPI). Radial velocity ambiguity resolution is directly determined. Additionally, any Doppler phase correction that is applied is applied in the correct unambiguous Doppler region, reducing the possible range and radial velocity errors.